MSX Infrared Dark Clouds and Massive Star Formation

Several thousand dark dust clouds have been found with the Midcourse Space Experiment (MSX) satellite that are so opaque they show up in silhouette against the Galactic background emission at 8 microns. They are also so cold that they show no emission at 100 microns, as observed by IRAS.

This image shows an MSX filamentary object about 20-parsec long, as seen with the SCUBA camera of the JCMT. The filament contains several potential starforming sites.

In 1999 April we used the JCMT to observe a selection of these MSX Infrared-Dark Clouds (IRDCs). Many of the dust clouds revealed in the SCUBA images have filamentary structures. Sometimes the filaments appear flocculent, with short, partially aligned segments. Bright spots, often unresolved, appear along the filaments. In all cases but one, these bright sources do not appear in the IRAS or MSX point source catalogs. Observations of CS 5-4, C34S 5-4, and HCO+ 3-2 were made of the brighter continuum sources discovered in the SCUBA images. These almost always showed line profiles indicative of outflow/infall. Several sources exhibited strong blue/red asymmetries, usually characteristic of infall. Most molecules (other than CO, CS, and HCO+) that have been observed to date show strong depletions relative to their abundances in normal molecular clouds.

The IRDCs consist of cold, dense dust and gas with temperatures less than 15 K, H₂ number densities >= 10⁶ cm^-3, and H₂ column densities ranging up to 10²³ cm^-2. From their kinematic distances, the filament extend in some cases over 20 pc and contain 10³-10⁵ solar masses. These structures resemble the large-scale dust clouds mapped in Orion by Johnstone and Bally (ApJL, January 1, 1999), but are even larger.

It is tempting to speculate that the IRDCs may be the long-sought centres of massive star formation in the Galaxy. The filamentary structures with bright spots along them bring to mind early theoretical star formation models involving collapse along magnetic fields (Mestel, 1977, in IAU Symp. 75).

Russell Redman        Russell started coming to the DAO as a volunteer in the summer of 1966 after finishing grade 9, with his first publication being the list of nearest stars in the 1970 Observer's Handbook. He has pioneered the study of minor planets at submillimeter wavelengths with the James Clerk Maxwell Telescope. Photo by Paul Feldman. Taken at the JCMT in April 1999, as the observations reported above were being obtained.

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